A number of (primary) RF spectrum users, such as, but not limited to, public service entities (e.g., police, fire, and utility service organizations), have been licensed by the Federal Communications Commission (FCC) to use one or more, relatively narrowband, radio channels, which were originally intended to support analog voice services (such as push-to-talk radio communications). In a typical spectral distribution of these narrowband channels, such as the non-limiting spectral distribution diagrammatically illustrated in the disjointed channel band 11 in FIG. 1, the channels that have been allocated to a given licensee will likely, over time, include a plurality of different bandwidth segments, such as one or more of 50 KHz, 25 KHz, 12.5 KHz and 6.25 KHz segments, which are not necessarily and are not expected to be mutually spectrally contiguous.
Namely, as shown by the spectral gaps 13, the channels which a primary user will have been licensed to use will typically be separated from one another by one or more other channels or bandwidth segments (gaps 13) that have been licensed to other (primary/licensed) users. The spectral disparity among the licensed channels results from the fact that they have been sequentially licensed to various users in response to incremental allocation requests, on the one hand, and due to the evolution of tighter spectral efficiency requirements that have been promulgated by the FCC to meet the continuously increasing demand for bandwidth.
A principal concern of these licensees is the efficient utilization of their allocated bandwidth. In the case of push-to-talk analog voice services, for example, a substantial number of licensees currently employ fixed-frequency, or manually channelized, radios. Although these radios are relatively inexpensive, they offer poor utilization of the radio channels where they use a dedicated frequency, or pair of frequencies; if the radio is used only ten-percent of the time, ninety-percent of the available bandwidth is wasted. In the above push-to-talk analog voice service example, additional radios could share the various channels of overall allocated segments of bandwidth by employing a ‘listen-before-talk’ user discipline. While this would improve spectral efficiency, it has the drawback of requiring some users to wait until a frequency becomes clear, or to manually adjust the frequency—if the radio has that capability—and try again.
Trunked radios offer an improvement over these stand-alone radio designs, since they are able to signal a repeater station, which then select a clear channel for the caller. While there are a number of trunking protocols that may be employed, they all share a disadvantage that is also shared by other push-to-talk mechanisms—channelization of the radios cannot be changed, and efficiency of band usage may be low.
The radios described above and other similar radios are inflexible, in that they can be used only for a single, relatively narrowband, channel (such as a 12.5 KHz, 25 KHz or 50 KHz channel), and must remain on that channel for the duration of the communication session—which prevents efficient utilization of the user's allocated bandwidth. Moreover, being relatively narrow bandwidth devices, these radios are unable to provide relatively high bandwidth services, such as Ethernet and Internet Protocol (IP) digital services.
As a result, in order to avail themselves of wideband services, users of such radios will often purchase cellular radio cards, which they can insert into their personal computers and thereby gain access to a cellular network that supports (IP) digital communications. Obvious shortcomings of this approach include the extra expense of the cellular card and service subscription, and having to rely upon a (cellular) network that may overload, or not function at all, in the event of an emergency situation, such as a natural disaster (e.g., hurricane), before, during and after which communications among emergency service organizations are critical.